The invention relates to a calcium carbonate pigment suitable for the coating of a material web and to a method for the production of a porous coating pigment. The invention also relates to cellulose-containing printing substrate which has a coating layer which contains calcium carbonate.
The pigments to which the invention relates are commonly used for the coating of a material web, such as paper or board. The printing substrates to which the invention relates are, for their part, used as substrates in forming text, pictures or other such symbols by adding printing ink onto the substrate. Typical printing techniques include techniques using liquid-containing printing inks, in particular inkjet printing.
1. Field of the Invention
Typical printing substrates include paper and board. The printing substrate may also be an inkjet printing paper. xe2x80x9cInkjet printing paperxe2x80x9d denotes paper which is used as the printing substrate when the inkjet printing technique is used. Inkjet printing paper may be, for example, a strongly or weakly stock-sized paper, a wood-free stock-sized or surface-sized paper or a paper developed specifically for inkjet printing.
Producing a good print quality by the inkjet printing technique sets both structural and physicochemical requirements on the surface part of the paper. The main objective regarding the paper is that ink should dry as rapidly as possible on the paper surface and should not penetrate into the fibre layer. Thus better colour density and contrast are achieved and, furthermore, print through is reduced.
The most important requirements set on successful inkjet printing are:
the ink must dry very rapidly on the surface of the printing substrate;
the density of the print must be high;
the side walls must be sharp;
dot gain must be controllable; and
the print must be well resistant to water, light and rubbing.
The most important property of inkjet printing paper in meeting the above-mentioned requirements in colour printing is ink absorbency, which includes absorption speed, absorption capacity and the orientation of absorption. The absorption speed must in particular in colour printing be so high that a preceding drop has time to become absorbed before the following ink drop is printed into contact with it. Thus the mixing of the colours can be reduced and the print quality be improved. The absorption time of a drop should preferably be at maximum a few milliseconds. High absorption capacity on the surface of paper for its part helps to bind a sufficient amount of ink in the vicinity of the paper surface. If too little ink is absorbed, the resistance to rubbing of the print decreases, and if the ink is absorbed too strongly, density is reduced and print through may increase. The orientation of absorption for its part affects the ink dot gain.
In single-colour printing, the requirements set on absorption properties are not as high as in colour printing. In single-colour printers the setting time for a drop may be up to 1-10 seconds, whereas in multi-colour printers a setting time of even less than 1 ms may be required in order that the ink drops should not become mixed. If the paper surface is sufficiently porous, rapid setting may be achieved, in which case the solvent in the ink drop does not have time to spread the drop too much and, also, successively printed drops will not become mixed together. The porosity of the paper surface should also be uniform in order that absorption should be as homogeneous as possible. The surface of the printing substrate should preferably have densely and evenly distributed narrow pores in the orientation of the substrate thickness.
In the manufacture of a printing substrate, it is known to apply, to the surface of the substrate, coating compositions which contain pigments in order to improve, for example, its printing and optical properties. In prior art there has not been available an inexpensive mineral-based pigment by means of which a sufficiently high absorbency on paper could be obtained economically.
Synthetic, silicate-based porous pigments have been used for increasing the absorbency of paper and board. These pigments are, however, quite expensive.
It is an object of the invention to eliminate the deficiencies described above and to provide a pigment of a completely novel type and a method for its production. It is also an object of the invention to provide a printing substrate of a completely novel type.
The invention is based on the idea that, in order to produce a pigment with good absorbency, porous calcium carbonate particles are produced through a degradation reaction from a crystalline initial material which contains the initial components for the forming of calcium carbonate and additionally initial components for the forming of reaction products to be removed from the initial material. In the reaction, a substantial portion of the calcium carbonate forms a porous frame in the space occupied by the initial material, around the open space left by the matter leaving the initial material. Calcium carbonate crystals, particularly calcite crystals which include capillary pores, are formed in the reaction. The invention is also based on the use of such porous calcium carbonate particles on the surface of a printing substrate as a pigment which absorbs printing inks or their solvents.
2. Description of Related Art
Porous calcium carbonate products have been disclosed, for example, in U.S. Pat. No. 5,007,964. These products, however, consist of agglomerated particles of calcium carbonate, resembling a bunch of grapes, or of aggregates which have been grown in several directions around the primary cores to form rosette-like crystal structures, as opposed to the particles according to the present invention, i.e. the unitary calcite crystals which include capillary pores.
According to the invention, it has been discovered that calcite crystals with desired structure are obtained as a result of a heat treatment, in which crystalline calcium oxalate is heated to a sufficient temperature for degrading it to calcium carbonate and to carbon monoxide. During a heat treatment, the temperature of the starting material is raised from a starting temperature to a target temperature, this temperature being maintained for at least two hours, after which the produced calcite is allowed to cool slowly, whereupon the desired capillary pores remain in the material. European Patent Application Publication No. 0 515 757 describes a method for the preparation of porous calcium carbonate particles having ink bottle-shaped pores therein. In this method calcium carbonate powder is..blended with sodium chloride, after which the powder blend is calcined at 900xc2x0 C. for producing calcium oxide particles. The calcium oxide particles are then slaked and carbonated, whereupon the desired product is achieved. The above described method is quite complicated and the structure of its end product does not correspond to the structure of the crystals according to the present invention.
More precisely, the pigment comprises porous crystals of calcium carbonate, having unitary crystalline frame of calcium carbonate around the open space defined by the pores, whereby the specific surface area of the crystals is at least 4 m2/g.
The method according to the invention is characterized in that calcium oxalate is heated in order to produce porous crystals of calcium carbonate. The printing substrate according to the invention is for its part characterized in at least 5% of the calcium carbonate particles are porous calcium carbonate particles having capillary pores and a specific area of at least 10 m2/g and a mass of 20-80% of the mass of a particle of a dense continuous calcium carbonate having the corresponding outer volume.
Considerable advantages are gained through the invention. Thus, with the help of the invention it is possible to produce pigment particles which are substantially made up of calcium carbonate and which have a higher pore volume than in prior art and thus a more advantageous specific surface area to mass ratio.
A preferred embodiment the invention has the additional advantage that the porous particles according to the invention increase the speed of ink absorption into the substrate surface as compared with calcium carbonate particles according to prior art. This has an advantageous effect in printing by techniques which utilize liquid-containing printing inks. One significant technique of this type is inkjet printing.
When the present invention is applied to inkjet printing substrates it is possible to improve the absorbency of the printing substrate and thereby improve the print quality, in particular in colour printing, in particular when soluble printing inks are used. In addition to soluble printing inks it is also possible to use in inkjet printing, for example, pigmented or so-called hot-melt inks. In colour printing, cyan, magenta and yellow are commonly used as colouring ingredients. The inkjet printing technique is also used in certain colour copiers, colour fax machines and multi-purpose machines which are, for example, combinations of a printer, a copier, a fax machine and a scanner.
In a preferred embodiment of the invention it is also possible to reduce the grammage of paper without weakening the other properties, such as strength or optical properties, of the paper.
The invention is directed to a calcium carbonate pigment suitable for the coating of a material web, the pigment comprising porous crystals of calcium carbonate, having unitary crystalline frame of calcium carbonate around the open space defined by the pores, whereby the specific surface area of the crystals is at least 5 m2/g. Preferably, the specific surface area is 10.0-10.0 m2/g.
The invention is also directed to a method for the production of a porous coating pigment, the method comprising heating calcium oxalate in order to produce porous crystals of calcium carbonate. In one embodiment of the method,
a material made up substantially of calcium oxalate is taken,
to produce pores, the material is heated to a target temperature in order to remove substantially the crystallization water bound in the calcium oxalate and the calcium oxalate is degraded to crystalline calcium carbonate and to carbon monoxide which leaves the material,
the target temperature is maintained at least for two hours, and
the crystals of calcium carbonate are allowed to cool after the treatment. In the method of the invention, the calcium oxalate can be degraded by heating it to a temperature above 440xc2x0 C. The calcium oxalate can be degraded by heating it in a substantially nitrogen or air atmosphere. The calcium oxalate can be heated to a temperature of 480-570xc2x0 C. The calcium oxalate can be degraded by heating it in a substantially carbon dioxide atmosphere. The calcium oxalate can be heated to a temperature of 510-800xc2x0 C.
The invention is also directed to a cellulose-containing printing substrate which has a coating layer which contains calcium carbonate, wherein at least 5% of the calcium carbonate particles are porous calcium carbonate particles having capillary pores and a specific surface area of at least 10 m2/g and a mass of 20-80% of the mass of a particle of a dense continuous calcium carbonate having the corresponding outer volume. The specific surface area of the porous particles can be at least 20 m2/g. The mass of the porous particles can be 40-70% of the mass of a particle of a continuous calcium carbonate having the corresponding outer volume. At least 10% of the calcium carbonate particles can be porous particles. At least 20-80% of the calcium carbonate particles can be porous particles. The cellulose-containing substrate can be a paper or board having a grammage of 30-500 g/m2. The printing substrate can be an inkjet printing paper. In addition to calcium carbonate particles and porous calcium carbonate particles, the coat can contain particles of at least one third kind. The coat can contain particles of calcium sulphate, aluminum silicate, kaolin, aluminum hydroxide, magnesium silicate, talc, titanium dioxide, barium sulphate and/or zinc oxide. The coat can contain synthetic pigment particles.
The invention is also directed to the use of a porous calcium carbonate produced by heating calcium oxalate as a substance absorbing printing ink or solvent on the surface of a cellulose-containing material web.
The invention will be examined below in greater detail with the help of a detailed description with reference to the accompanying drawings.